Often you need to inject into the flight plan an along track waypoint, that is defined by a distance from a known waypoint or navigational aid. There are several ways to achieve this and each method is similar in operation, but is used in differing circumstances. Depending upon the software version installed in the Control Display Unit (CDU), you can either use the LEGS or FIX page.
To Create a Place Bearing Waypoint and a Along Track Waypoint in the CDU
There are two methods that obtain similar results - a Place Bearing Waypoint (Points 1 through 3) and an Along Track Waypoint.
- In the LEGS page, type into the scratchpad the waypoint name, vector and distance. The distance is where the new waypoint will be created and is measured from the waypoint or navigation aid you are using as an anchor (LAV340/10).
- After entering the information into the scratchpad (LAV340/10), upload to the CDU by key pressing the appropriate waypoint, where you wish the along track waypoint to be created.
- Clean up any discontinuity and execute.
- LAV is the waypoint, 340 is the vector, in degrees, that the waypoint will be created, And 10 is the distance from LAV that the new waypoint will be created.
- To insert the waypoint before the anchor waypoint, use the negative key - (LAV340/-10).
- To insert the waypoint after the anchor waypoint, do not use any identifier (LAV340/10).
- Note that the / is after the vector, and the waypoint name and vector are joined with no spaces.
To insert an Along Track Waypoint the identifier and distance is typed into the scratchpad and up-selected. For example, LAV/-10 or LAV/20.
If the target waypoint is already in the route, then it is not necessary to type the identifier into the scratchpad; rather, select the identifier from the route list and press the appropriate line select. The software will automatically insert the selected waypoint into the scratchpad for editing.
If you want the new waypoint to be created along the current navigation track, type the vector that is displayed adjacent to the waypoint in the LEGS page of the CDU. If you do not type a vector, then the location of the along track waypoint will be automatically assigned by the software, which may not be along your current track course.
In the example shown (LEFT), I have created an along track waypoint called LAV02 which is located the aircraft side of the waypoint LAV. I also have created a circle around LAV (see instructions below).
The CDU software will create along track waypoints with generic names (LAV01, LAV02, LAV03, etc). In the example, I have already flown over LAV01 and LAV02 is now the next waypoint.
METHOD 1 - How To Create a Circle around a Waypoint Using the FIX Functionality in the CDU
There are many reasons why you may want to place a circle identifier around a waypoint or navigational fix at a specific distance. Least of which to highlight a distance from a waypoint so it can easily be seen on the Navigation display (ND). Other reasons are to designate the distance from the runway threshold that the landing gear is to be lowered, or the beginning of the glideslope for an ILS approach.
- Press FIX on the CDU to open the FIX page.
- Type into the CDU scratchpad, the name of the waypoint or navigation aid (VOR, NDB, etc) and upload to the FIX page (in the example, LAV). This will display a small circle around the identifier in the Navigation Display in green-dashed lines.
- Type into the scratchpad the distance you require the circle to be drawn around the waypoint. For example, /15 and upload this to LSL1.
A quick way to insert a waypoint from a route into the FIX page is to press the waypoint name in the LEGS page. This will automatically down select the waypoint to the scratchpad saving you the time typing the name. Then select FIX and upload the waypoint from the scratchpad.
In the example shown (ABOVE), I have created a circle at a distance of 15 miles from LAV which can be viewed on the navigation Display (ND). The straight, green dashed line displayed from LAV is the vector, which in this example is unimportant, as we are only creating a circle.
The vector only becomes important when you want to create a waypoint that intersects the edge of the circle (see below).
METHOD 2 - How to Create a Single Along-Track Waypoint (at the edge of the circle)
If you want to create an along track waypoint at the edge of the defined circle, and insert this waypoint in line with the aircraft's current navigation track (route), then the vector becomes more important. The vector will determine the bearing, from the waypoint anchor that the waypoint is created.
A waypoint can be created anywhere along the circle, that is drawn at the nominated distance around the anchor waypoint. The location of new waypoint is defined by the vector (bearing). If the inserted waypoint is in line with aircraft's current navigation track, the vector shown in the FIX page or adjacent to the waypoint in the LEGS page should be used. The new waypoint will be displayed before the anchor waypoint.
- Open the FIX page and type into the scratchpad the waypoint or navigation aid identifier and up-select to the four digit space at the top of the screen (for example, LAV). If the identifier is part of a route, open the LEGS page, select the waypoint from the route and press the appropiate line select button for the waypoint which will populate the scratchpad. Then, open the FIX page and up-select from the scratchpad to the FIX page the waypoint. A dashed-green coloured circle will be displayed around the waypoint in the Navigation Display.
- Type into the scratchpad the appropriate bearing vector and distance and up-select to the appropriate line (for example, 049/5 will generate a waypoint 5 miles from the anchor waypoint on a bearing of 049 degrees). A dashed-green coloured circle will be drawn at 5 miles from the anchor waypoint, and the new waypoint will be shown along that line at the appropriate bearing.
- To place this new waypoint into the route, press the appropriate line select button adjacent to the entry and down-select this to the scratchpad. The CDU software will automatically convert the entry to a format that can be inserted into the route.
- Open the LEGS page and up-select the waypoint to the location within the route that you wish the waypoint to appear.
- Finally, clean-up any discontinuity (DISCO).
In the example (BELOW), I have created the first, along track waypoint (ADM 01) as an entry point to an arc, directly in line with the navigation track of the aircraft, before reaching the anchor waypoint - ADMAR. ADM 01 is at the edge of the defined circle.
METHOD 3 - How to Insert Additional Along-Track Waypoints Around the Arc of the Circle
Another variable of the above theme, often used when executing an NDB Approach, is to create several along track waypoints that intersect a defined circle. The waypoints which are created at a set distance from an anchor waypoint or navigation fix, create an arc.
First, ensure you have a circle created around the waypoint at the distance required (FIX page).
- Select the anchor waypoint for the arc from the LEGS page and download to scratchpad.
- Type in the scratchpad as seperate entries: ADMAR100/15, ADMAR130/15, ADMAR160/15, ADMAR190/15 and so forth and up-select to the appropiate line select in the route. This will create an arc 15 miles from ADMAR at the different bearing locations.
- If you want the first waypoint to be along your navigation track, use the vector for this initial waypoint as indicated in the LEGS page of the CDU.
- The above can also be generated from the FIX page using the methods outlined in Method 2.
In the example (LEFT), I have created a number of along track waypoints, separated each by 30 degrees, that intersect the circle 10 miles from ADMAR. With a little imagination, it is easy to uncover several uses for such funtionality: terrain avoidance, approach protocols, etc. Click image to enlarge.
It is important to note that user and along track waypoints are given generic names by the software (ADMAR01, ADMAR02, ADMAR03, etc).
To ensure that the waypoints are sequential when displayed (01,02,03,04,05, etc), upload the new waypoints to the LEGS page, to the same waypoint name that was used to create the along track waypoint. In this example it is ADMAR.
Understanding the CDU
What I have described above is but a very brief and basic overview of some functions that are easily performed by the CDU.
CDU operation can appear to be a complicated and convoluted procedure to the uninitiated. However, with a little trail and error you will soon discover a multitude of uses. It is important to remember, that there are often several ways to achieve the same outcome, and available procedures depend on which CDU software you are using.
I am not a professional writer, and documenting CDU procedures that is easily understood is challenging. If this information interests you, I strongly recommend you purchase the FMC Guide written by Bill Bulfer. Failing this, navigate to the training section of this website and download some of the FMC tutorials.
Anchor Waypoint – The waypoint from which additional waypoints are created from
CDU – Control Display Unit
FMC – Flight Management Computer
Target Waypoint – The waypoint that has been generated as a sibling of the Anchor waypoint
Waypoint – Navigation fix, usually an airport, VOR, NDB or similar